The audio feedback effect, also called microphone feedback, occurs when a sound wave enters a microphone having a frequency that is the same as the frequency of a sound wave at an output of the microphone.
Feedbacks could happen on the electronic equipment which receives and broadcasts sounds. When the External Feedback Path is formed, where sound waves generated by the broadcast point are received by the collecting point, sound waves are thus constantly repeatingly amplified.    There are 2 major impacts of feedbacks.
1. When feedback sounds are mixed with the original sounds, it would cause acoustic distortion.
2. When feedbacks of the same frequency repeatingly accumulate, and volume gain is too large, piercing whistles occur.
Cancellations in High Fidelity Acoustics:
(1) A microphone cannot determine whether the incoming sounds or signals are from an objective sound source or from noises, such as background noises or internal microphone generated noises. When objective sounds are interfered with by noises, their sound waves are changed, and thus the acoustic quality is affected.
(2) Traditional noise filters can solve this issue by treating the frequency of the incoming signals. If the noise and the sound source's frequencies are different, a high-pass filter (which allows only sounds below certain frequency to pass), a low-pass filter (which allows only sounds above certain frequency to pass), or a range-pass filter (which allows only sounds within certain frequency range to pass) can be used to filter out the noise.
(3) However if the noise and the objective sound's frequencies are the same, or are close (such as multiple reflections of the objective sound), the objective sounds and noises are similar, and the filter cannot delete the noise.
(4) In addition, irrespective of whether digital or analogue filters are used, or if frequency or time-domain filters are used, all are more-or-less subjected to mathematical transformations. The transformations result from distortion and time delay issues. Thus the better a filter is, the more complex design and mathematical conversions are required. For example the latest Wavelet filter could be used, but it is very expensive.